This invention relates to the processing of uranium. More particularly, this invention relates to a process for producing enriched uranium in the form of UF.sub.6, with low assay metallic tails, using a combination of a gaseous diffusion process (or a gas centrifuge process) and an atomic vapor laser isotope separation process.
Enriched uranium, i.e., uranium having a .sup.235 U content higher than the naturally occurring 0.7 wt. %, has been produced for many years using the well known gaseous diffusion process in which .sup.235 U/.sup.238 U isotopes, in the form of UF.sub.6, are separated from one another by passing the UF.sub.6 vapors through multiple porous membrane filter stages in which the transmitted fraction, which is fed into the primary side of the next stage, is richer in .sup.235 U than the retained fraction, which is fed back to the primary side of the preceding stage. While this process can produce a .sup.235 U/.sup.238 U isotopic mixture of almost any ratio (by providing a sufficient number of membrane stages), the process is very energy intensive and produces UF6 tails which present an ever growing storage and disposal problem.
More recently a newer process has:been developed for uranium enrichment, using metallic uranium instead of UF.sub.6 gas as the feed material. This process, known as Atomic Vapor Laser Isotope Separation (AVLIS), is described by Benedict, Pigford, and Levi, in a book entitled "Nuclear Chemical Engineering", published in 1981 by McGraw-Hill Book Company (New York), where uranium isotope separation is described in general at pages 812, 813, and 817, and laser isotope separation of uranium metal vapor (the AVLIS process) is specifically discussed and described on pages 914-919. This Atomic Vapor Laser Isotope Separation process is also described by J. A. Paisner in an article entitled "Atomic Vapor Laser Isotope Separation", published in Applied Physics B, Volume 46, at pages 253-260 (1988), the disclosure of which is hereby incorporated by reference herein.
Briefly, the process consists of vaporizing an isotopic mixture of .sup.235 U/.sup.238 U in metallic form, and then selectively ionizing the .sup.235 U isotope using a laser energy source such as a copper laser. The ionized .sup.235 U is collected on a negatively biased electrode as the desired product stream, while the non-ionized .sup.238 U vapors pass out of the extractor as the tails stream. This process uses substantially less energy than the traditional gaseous diffusion separation process and produces a lower .sup.235 U content tail in metallic form which is preferable to the UF.sub.6 tail conventionally produced in the gaseous diffusion separation process from a storage and disposal standpoint.
It would, therefore, appear to be desirable to perform all uranium enrichment by the atomic vapor laser isotope separation process. From a standpoint of process economics, however, the atomic :vapor laser isotope separation process is most desirably applied for enrichment levels in the order of 2-5 wt. %, and typically about 2-3 wt. %. There are certain applications, however, where a higher .sup.235 U content would be desirable, such as, for example, light water reactors needing uranium with assays of 4 to 5 wt. % .sup.235 U, research reactors, naval reactors, and high temperature gas cooled reactors needing a 93.5 wt. % .sup.235 U as described by Benedict, Pigford, and Levi, on page 148 in "Nuclear Chemical Engineering". Conventionally such needs for such higher .sup.235 U content have necessitated the use of the gaseous diffusion separation process.
It would, however, be desirable if one could somehow produce an enriched uranium having at least 4 wt % .sup.235 U, such as is available with the gaseous diffusion separation process, while still enjoying the benefits of lower energy consumption and production of lower .sup.235 U assay uranium tails in metallic form which characterize the atomic vapor laser isotope separation process.